1. Field of the Invention
The present invention is directed to a printed circuit board having an interstitial component and a method of making the same.
2. Description of the Related Art
Conventional printed circuit boards (xe2x80x9cPCBsxe2x80x9d) require more space as components are added. Such components include integrated circuits (xe2x80x9cICsxe2x80x9d), application specific integrated circuit (xe2x80x9cASICxe2x80x9d) chips, diodes, transistors, resistors, capacitors, thermocouples, and the like. In addition, the lack of space on the surface of the PCB can prevent the optimal placement of electrical components, such as sensing components, with respect to the chip or IC being monitored.
For example, thermocouples can be employed in two conventional ways to monitor an ASIC on a PCB. The first implementation is to glue a thermocouple onto the top of the ASIC with long wires soldered to the board. A second conventional implementation is to solder a small thermocouple (either a monolithic part or a discrete part) to the PCB some distance from the ASIC.
The first implementation requires significant (and expensive) post-processing by hand after the PCB has been loaded by a machine. The soldering and the gluing must be done by a technician. This implementation results in accurate temperature sensing because the thermocouple makes direct contact with the ASIC.
The second implementation results in the thermocouple being placed some distance from the device it is trying to measure. This approach is most commonly employed, since the parts can be placed by machine and thus minimize the associated costs. However, a problem with this second approach is that it leads to inaccurate results, due to potential air movement between the device and the thermocouple.
A current method for connecting coupling and terminating components is to run traces on internal layers of the PCB and then use a via or multiple vias to bring the signal to the surface. The signal goes through the appropriate component and then through another via back down to an inner signal layer. However, there are several problems with this conventional implementation.
The first, and most important problem, is that the vias and the above-board components cause perturbations in the impedance of the trace. Such perturbations cause voltage and current reflections in the trace due to the impedance mismatch. Secondly, when high-speed signals reach the outer layers of the PCB, the electromagnetic energy that is radiated can no longer be contained by the board itself. This electromagnetic energy can cause electromagnetic interference (EMI) problems and may increase the engineering time required to solve electromagnetic compatibility (EMC) problems associated with the EMI. EMI and EMC are important issues, since there are regulatory bodies that specify how much EMI can come from a product. For example, product shipment can be stopped due to EMI emissions failures.
In view of the foregoing, according to an embodiment of the present invention, a printed circuit board (PCB) comprises a first layer having first and second surfaces, with a device mounted on the first surface. The PCB also includes a second layer having third and fourth surfaces, with the fourth surface being the bottom of the PCB. One of the second and third surfaces can include a recessed portion for securedly holding an interstitial component. A via, electrically connecting the PCB layers, is coupled to a lead of the interstitial component.
According to yet another embodiment of the present invention, a method of making a printed circuit board having an interstitial component, and also including a first substrate layer having first and second surfaces and a second layer having third and fourth surfaces, comprises routing a portion of one of the second and third surfaces a layer surface to form a recessed portion. An interstitial component is placed in the recessed portion and a solder paste compound is applied to a surface-mount axial lead of the interstitial component to couple the lead from the interstitial component to an inner trace of the printed circuit board.
With the apparatus and methods according to several of the embodiments of the present invention, distinct advantages over conventional devices can be achieved. For example, in conventional structures the placement of devices, especially passive devices, was implemented by placing them on either side of a PCB. With the PCB according to the present invention, such components can be placed inside the PCB. There are several advantages to this type of PCB construction. First, the PCB according to several of the embodiments of the present invention eliminates components from the top and bottom of the PCB, thus freeing space for larger components such as ASICs. Further, the PCB design allows interstitial components to be placed in optimal position in relation to aboveboard components that are being monitored or are sending signals. For example, the PCB design according to the present invention allows certain components to be optimally placed in relation to high speed signal traces such that signal perturbations and potential EMI radiation are significantly reduced. Other advantages include the possibility of tuning the PCB substrate cavity to further enhance signal quality and reducing the number of signal vias that must traverse the PCB.
Further features of the invention form the subject matter of the claims and will be explained in more detail, in conjunction with further advantages of the invention, with reference to exemplary embodiments.